A conventional drive assembly for controlling a powered device, e.g., a valve such as a slide valve or clack valve, has a motor and a transmission between the motor and the powered device. The transmission has a hollow gear which is provided with internal teeth for establishing a connection to the motor. The gear is also formed with external teeth which are designed so that the gear can function as a worm gear. The external teeth permanently mesh with a worm of a handwheel, i.e., with a screw thread on the shaft of the handwheel, and the gear is held in engagement with the worm by a self-locking action. When the motor operates, the worm holds the worm gear stationary.
Drive assemblies of this type have proved successful. In such a drive assembly, the motor should shut off automatically when the powered device operated by the motor generates an excessive reaction force as occurs, for instance, upon encountering an obstruction. Furthermore, it can be desirable to detect a torque increase in the transmission. For example, an increase in torque can indicate changes in a medium flowing through the powered device or increasing wear in the drive assembly.
To this end, the worm in mesh with the worm gear is resiliently mounted for axial displacement. When the worm has shifted a predetermined distance, a limit switch, potentiometer or inductive distance recorder is activated.
Only a single point can be detected with a limit switch. On the other hand, although a potentiometer permits more than one point to be detected, relatively large displacements of the order of millimeters are required. The same applies for distance recorders. Moreover, all of these mechanisms yield an indirect measurement of torque or force. This is so because a reaction force is indicated by means of springs in response to axial shifting of the worm.
A substantial drawback of these indirect measuring units is that relatively large displacements of the worm are required. In addition, the measurements are a function of the spring constants which can vary, e.g., as the springs age. Furthermore, too large a reaction force is indicated upon fracture of a spring since a larger displacement then occurs.
The conventional drive assemblies also have the drawback that many coupling elements are required between the displaceable worm and the associated measuring, indicating and control units. The large number of coupling elements is not only costly but takes up a considerable amount of space and can result in inaccuracies due to tolerances and friction. Further, these inaccuracies increase over time because the coupling elements undergo wear.